Dilute solutions (50 ng/ml) of apomorphine in plasma are unstable at 37 degrees C and pH 7.4. The chemical half-life is only 39 min. Mercaptoethanol (0.01%) is effective in stabilizing these samples while sodium metabisulphite (1%), which is generally used, is not effective. Biological samples are extracted with diethyl ether (recovery 96.5%) and analysed using HPLC with coulometric detection (oxidation potential 0.25 V). The stationary phase employed was C18 material (4 microns) and the mobile phase was phosphate buffer (pH 3)-acetonitrile (70:30, v/v). The flow-rate was 1.8 ml/min. This bioanalytical method presents a reliable tool for pharmacokinetic studies in man.